1. Left 4 Dead 2
A majority of shooters add co-op in as an afterthought. A second nameless character is dropped into the world, given life just to act as the embodiment of utility. Valve's Left 4 Dead series denies this reality and substitutes its own: a game built from the ground up for four players to fight side-by-side to survive the zombie apocalypse. Every stage is made endlessly replayable, and populated with foes designed specifically to force players to play together to the best of their ability. While the first game was strong, including brilliantly designed elements specifically made to enhance the co-op experience, Left 4 Dead 2 really perfected the formula, shining the survival-based gameplay into a nugget of solid gold.
Mechanically it wasn't all that different from the first - it was just better. The levels were better, the weapons were more interesting, and the characters were were every bit as memorable as the original quartet. From Coach's awesome vocals in the Midnight Riders level to Ellis's adorably inane stories, we felt completely connected to Nick, Ellis, Coach, and Rochelle, leading to months lost playing with our friends and attempting to survive the end of the world.